Distributed generation (DG) systems are becoming increasingly attractive for a number of reasons. DG systems are often based on renewable energy resources such as sun, wind and water and thus reduce the amount of greenhouse gasses and other pollutants, help protect against possible shortages of power and outages, provide a more economical solution for remote areas due to transmission costs, reduce transmission system losses and upgrade rates, may offer combined heat and power (CHP) solution to customers, and reduce dependency on fossil fuels.
Electronic power converters are widely used to interface DG systems with the utility grid. Such an interface is equipped with control/sychronization strategies to ensure that controlled power is extracted from the primary source and transferred to the grid without violating the grid codes and standards such as CSA-C22.2, UL 1741, IEEE 1547, and IEC 62109-1. The AC power is typically controlled by controlling the active and reactive powers separately and by controlling the current injected into the grid. The injection of active power is often the main objective in a DG system but it can also provide reactive power to the local load if required.
In a three-phase system, the active and reactive powers can be conveniently controlled using the concept of dq rotating synchronous reference frame (SRF). The dq components of the current signals are DC variables that are proportional to active/reactive powers. Thus, simple proportional-integrating (PI) controllers together with decoupling terms can be used to control those variables.
In single-phase applications, the current dq components can also be generated using αβ-dq transformation where the same three-phase current control strategy can be applied. In such approaches, however, the β component is not externally available and needs to be synthesized using a ninety-degree phase-shift operation at the fundamental frequency. The ninety-degree phase-shift operation can be performed by different methods such as time-delay, all-pass filter, Hilbert transform, second-order generalized integrator (SOGI), or an enhanced phase-locked loop (EPLL). In addition to the challenges involved in accurate and efficient realization of the phase-shift operation, its dynamics strongly contribute to a decrease in the speed and the stability margins of the control system.
Another class of power control strategies for single-phase applications is based on performing the control at the fundamental frequency using a proportional-resonant (PR) controller. The current reference is generated as a pure sinusoidal signal whose amplitude and phase angle are controlled. In one approach, which is widely used in multi-stage topologies, to balance the input power extraction with output power injection, the DC link voltage is regulated to a desired value, which results in a reference for the magnitude of the output current. The angle of the current is synchronized with the grid voltage using a PLL.
However, conventional single-phase systems typically exhibit one or more drawbacks, such as slow system response, computational complexity, sensitivity of the control algorythms to system uncertainties and varied operating conditions, and complexities in dealing with harmonics.